1. Technical Field
The present invention relates to a semiconductor device having a light receiving element on a substrate.
2. Related Art
Reproduction and recording by currently-disseminated CD (compact disk) and DVD (digital versatile disk) take place by irradiating laser beam on recoding media such as CD and DVD, and converting the reflected light from the recording media into electric signals by a light receiving element.
Only a few scattering would occur if surface condition of the recording media is smooth. The surface may, however, be roughened, scratched, deformed or get dust adhered thereon. In these cases, scattered light would occur due to non-smoothness of the surface, so that the light may enter circuit portions (composed of elements including transistor, capacitor, resistor and so forth) other than a photodiode portion of a light receiving element, and may be causative of malfunction of the circuit.
State of use of the recording media is not uniform, and this makes it difficult to keep smooth surfaces of the media. It is, therefore, necessary to develop a light receiving element not causative of malfunction even under incidence of scattered light.
In order to prevent the scattered light from entering, any unnecessary influences of the carriers, generated outside the region where the photodiode is formed, should be prevented. For this reason, the surface of the semiconductor circuit is covered with a light-shielding film composed of a metal interconnect layer such as an aluminum layer. Parasitic capacitance may, however, be arisen by the light-shielding film in the light receiving element, and may adversely affect the frequency characteristics over a wide frequency range.
Reasons for the drawback resides in generation and diffusion of carriers in the substrate, induced by light irradiated to portions of the substrate not shielded from light, such as side faces. Accordingly, the carriers migrate to the bipolar transistor and the capacitor. The migrated carriers flow through the circuit portion without being interrupted, and contribute as electric current. As a consequence, the current flows through the circuit portion, adversely affect the circuit operations, and may be causative of malfunction.
Japanese Laid-Open Patent Publication No. 2003-69071 describes that a PN junction is formed over the back surface of a silicon substrate as being opposed to the photodiode portion, so as to allow the PN junction portion to absorb unnecessary carriers, to thereby re-combine false carriers with the aid of electrodes on the back surface. In this way, the carriers are prevented from generating at the junction portion, and thereby the diffusive components may be suppressed.
Japanese Laid-Open Patent Publication No. 2002-217448 describes a semiconductor luminance sensor having PN junctions formed in the surficial portion under the photodiode portion and circuit portion, allowing the junctions to absorb unnecessary carriers. In this sensor, an NPN bipolar transistor is formed over a uniform P-type silicon substrate.
However, the conventional techniques described in the above literatures have had still some room of improvement in the aspects below.
As for Japanese Laid-Open Patent Publication No. 2003-69071, efficiency of absorption of unnecessary carriers may vary depending on wavelength of the incident light, so that malfunction as being affected by the unnecessary carriers may still occur if the device is used under a condition covering a broad wavelength band ranging from the visible to longer (600 nm to 1000 nm) region. In addition, the carriers generated by light incident on portions other than the photodiode portion may be absorbed only to a less efficient degree. The device is therefore restricted in that light should be incident on the photodiode portion.
Japanese Laid-Open Patent Publication No. 2002-217448 relates to a structure having the PN junctions formed in the surficial portion under the photodiode portion and circuit portion, allowing the junctions to absorb unnecessary carriers. The carriers generated under the bipolar transistor may have a tendency of being attracted by electric field of the depletion layer, but any other diffusive carriers may not be interrupted. The uninterrupted carriers may diffuse into the base region, induce electric current, and adversely affect the circuit.
The present invention is conceived after considering the above-described situation, and is to prevent the carriers, generated by incidence of light through the surface of the substrate, from diffusing into the circuit portion.